1. Field of the Invention
The present invention relates to boron and aluminum complexes, to the preparation thereof and to the use thereof for solubilizing ionic compounds.
2. Description of Related Art
Many compounds of use for performing nucleophilic substitutions which make it possible to modify materials and molecules in inorganic chemistry or in organic chemistry are known. Mention may in particular be made of compounds which have an F−, OCN−, O2−, O22−, O2•−, OH−, RO−, N3−, CN−, HNCN−, or NCN2− anion. Fluorinated compounds and isocyanates are particularly advantageous for the polymer or refrigeration agent industry, for surface treatment, or for pharmaceutical or agrochemical products. However, the nucleophilic substitution reactions using these compounds are impossible in protic solvents, such as, for example, H2O, CH3OH, formamide and N-methylformamide, which are capable of dissolving the salts of said anions, since the very strong solvation of the abovementioned anions reduces their nucleophilic nature. In addition, when a compound of one of the abovementioned anions is brought into contact with lamellar inorganic structures of FeOC1 or TiNCl type, there is a greater risk of exfoliation owing to penetration of the ion in solvated form between the sheets. In polar aprotic solvents, the reactivity of the abovementioned anions is limited by the very low solubility of the corresponding alkali metal or alkaline-earth metal salts. Another problem comes from the basicity of said anions, the charge of which is concentrated in a small volume (rF−=rOH−=rO2−=1.4 Å), which promotes the reaction in which protons in the α-position are eliminated in organic chemistry reactions, thereby prohibiting substitutions on sensitive substrates. The elimination in the β-position (Hoffmann degradation) limits the stability of the quaternary ammonium cations on quite long chains so as to induce solubility in aprotic solvents.
Catalysts which are more thermally stable than quaternary ammoniums have been proposed, in particular in U.S. Pat. No. 7,217,842 or by A. Pleschke, et al. Röschenthaler Journal of Fluorine Chemistry, Volume 125, No. 6, June 2004, pages 1031-1038. These onium salts only minimally reduce the temperature at which Halex reactions are carried out and do not solve the problem of eliminations on sensitive substrates.
One proposed solution for increasing the solubility of the alkali metal or alkaline-earth metal salts of the abovementioned anions in aprotic solvents consists in complexing the alkali metal or alkaline-earth metal cations with a complexing agent chosen, for example, from diglymes, triglymes, tetraglymes, crown ethers and cryptates. The use of these agents actually makes it possible to increase the solubility of the above-mentioned salts in aprotic solvents. However, the rate of the elimination reactions is greatly increased and, consequently, these additives are only of slight interest. In addition, the most effective complexing agents for increasing solubility are expensive compounds, in particular crown ethers and cryptates. As a result, the use of complexing agents has not given rise to major industrial developments.
In practice, exchange reactions between chlorine and fluorine are carried out at very high temperatures in solvents such as sulfolane (tetramethylenesulfone). For example, with potassium fluoride (Halex process), the exchange reaction is carried out at temperatures of from 200° C. to 300° C., with low yields in terms of materials and high energy costs.
No industrial preparation of isocyanates from metal cyanates or industrial preparations from O2−, O22−, O2•−, HNCN− and NCN2− anions are known. The OH−, RO−, CN−, and N3− anions are used for nucleophilic substitution reactions, but the yields are low owing to the competing elimination reactions. It is known that Lewis acids such as BF3 and PF5 are capable of complexing fluorine ions so as to give the coordination anions BF4− and PF6−, the corresponding metal salts of which are readily soluble in polar aprotic solvents. However, the salts of the BF4− and PF6− anions have no nucleophilic power for performing substitutions, for example, of a chlorine atom with a fluorine atom, owing to the large amount of energy of the Lewis acid a base interaction. On the contrary, the salts of these anions are used when a strictly non-nucleophilic and non-complexing anion is required, in order to stabilize unstable cationic species or as a support electrolyte in electrochemistry. Also known are triphenylboron derivatives, in particular the fluorinated derivative (C6F5)3B which is a powerful Lewis acid capable of complexing LiF despite the high reticular energy of this salt. (C6F5)3BF− anions, like BF4− and PF6−, are not nucleophiles allowing fluorine exchange. In addition, these compounds are difficult to obtain and they are extremely expensive.
U.S. Pat. No. 2,909,560 describes a process for preparing a cyclic boron compound, 2,6,7-trioxa-1-bora-4-methylbicyclo[2.2.2]octane, by reacting 1,1,1-trimethylolethane (I) with boric acid H3BO3 (II), and the use of said cyclic compound as a stabilizer of polyethylene.